The exponential decay of the strength of magnetic fields dictates that, in induction welding processes, the structure closest to the induction coil will be the hottest, since it experiences the strongest field. Therefore, it is difficult to obtain adequate heating at the bond line between two graphite or carbon fiber reinforced resin matrix composites relying on the susceptibility of the fibers alone as the source of heating in the assembly. For the inner plies to be hot enough to melt the resin, the outer plies closer to the induction coil and in the stronger magnetic field are too hot. The matrix resin in the entire piece of composite melts. The overheating results in porosity in the product, delamination, and, in some case, destruction or denaturing of the resin. To avoid overheating of the outer plies and to insure adequate heating of the inner plies, a susceptor of significantly higher conductivity than the fibers is used to peak the heating selectively at the bond line of the plies when heating from one side. An electromagnetic induction coil on one side of the assembly heats a susceptor to melt and cure a thermoplastic resin (also sometimes referred to as an adhesive) to bond the elements of the assembly together. Often the current density in the susceptor is higher at the edges of the susceptor than in the center because of the nonlinearity of the coil. This problem typically occurs when using a cup core induction coil like that described in U.S. Pat. No. 5,313,037 and can result in overheating the edges of the assembly or underheating the center, either condition leading to inferior welds because of non-uniform curing. It is necessary to have an open or mesh pattern in the susceptor to allow the resin to bond between the composite elements of the assembly when the resin heats and melts.
Three major joining technologies exist for aerospace composite structure: mechanical fastening; adhesive bonding; and welding. Both mechanical fastening and adhesive bonding are costly, time consuming assembly steps that introduce excess cost even if the parts that are assembled are fabricated from components produced by an emerging, cost efficient process. Mechanical fastening requires expensive hole locating, drilling, shimming, and fastener installation, while adhesive bonding requires complicated surface pretreatments.
In contrast, thermoplastic welding, which eliminates fasteners, features the ability to join thermoplastic composite components at high speeds with minimum touch labor and little, if any, pretreatments. In our experience, the welding interlayer, called a susceptor, also can simultaneously take the place of shims required in mechanical fastening. As such, composite welding holds promise to be an affordable joining process. For "welding" thermoplastic and thermoset composite parts together, the resin that the susceptor melts functions as a hot melt adhesive. If fully realized, the thermoplastic-thermoset bonding will further reduce the cost of composite assembly.
There is a large stake in developing a successful induction welding process. Its advantages versus traditional composite joining methods are:
reduced parts count versus fasteners PA1 minimal surface preparation, in most cases a simple solvent wipe to remove surface contaminants PA1 indefinite shelf life at room temperature PA1 short process cycle time, typically measured in minutes PA1 enhanced joint performance, especially hot/wet and fatigue PA1 permits rapid field repair of composites or other structures.
There is little or no loss of bond strength after prolonged exposure to environmental influences.
U.S. Pat. No. 4,673,450 describes a method to spot weld graphite fiber reinforced PEEK composites using a pair of electrodes. After roughening the surfaces of the prefabricated PEEK composites in the region of the bond, Burke placed a PEEK adhesive ply along the bond line, applied a pressure of about 50-100 psi through the electrodes, and heated the embedded graphite fibers by applying a voltage in the range of 20-40 volts at 30-40 amps for approximately 5-10 seconds with the electrodes. Access to both sides of the assembly is required in this process which limits its application.
Prior art disclosing thermoplastic welding with induction heating is illustrated by U.S. Pat. Nos. 3,996,402 and 4,120,712. In these patents, the metallic susceptors used are of a conventional type having a regular pattern of openings of traditional manufacture. Achieving a uniform, controllable temperature in the bondline, which is crucial to preparing a thermoplastic weld of adequate integrity to permit use of welding in aerospace primary structure, is difficult with those conventional susceptors, as we discussed and illustrated in our copending U.S. patent application Ser. No. 08/068,520.
Thermoplastic welding is a process for forming a fusion bond between two faying thermoplastic faces of two parts. A fusion bond is created when the thermoplastic on the surface of the two parts is heated to the melting or softening point and the two surfaces are brought into contact, so that the molten thermoplastic mixes, and the surfaces are held in contact while the thermoplastic cools below the softening temperature.
Simple as the thermoplastic welding process sounds, and easy as it is to perform in the laboratory on small pieces, it becomes difficult to perform reliably and repeatably in a real factory on full-scale parts to build a large structure such as an airplane wingbox. The difficulty is in getting the proper amount of heat to the bondline without overheating the entire structure, and also in achieving intimate contact of the faying surfaces of the two parts at the bondline during heating and cooling despite the normal imperfections in the flatness of composite parts, thermal expansion of the thermoplastic during heating to the softening or melting temperature, flow of the thermoplastic out of the bondline under pressure, and then contraction of the thermoplastic in the bondline during cooling.
One technique for getting heat to the bondline in a thermoplastic assembly to be welded together is to include a conductive layer or article, known as a "susceptor", between the two surfaces to be welded, and to heat the susceptor by resistive heating so that the susceptor functions as a bondline heat source to melt or soften the thermoplastic at the bondline for fusion of the faying surfaces of the composite components to be joined. The electric current for heating the susceptor can be in the form of eddy currents generated inductively, as taught for example by U.S. Pat. Nos. 3,395,261 and 4,978,825, or it can be conducted directly to the susceptor through tabs or the like as shown in U.S. Pat. No. 5,313,034.
In U.S. patent application Ser. Nos. 08/286,360 and 08/068,520, we described a tailored susceptor for approaching the desired temperature uniformity. This susceptor relied upon carefully controlling the geometry of openings in the susceptor (both their orientation and their spacing) to distribute the heat evenly. For example, we suggested using a regular array of anisotropic, diamond shaped openings with a ratio of the length (L) to the width (W) greater than 1 to provide a superior weld over that achieved using a susceptor having a similar array, but one where the L/W ratio was one. By changing the length to width ratio (the aspect ratio) of the diamond-shaped openings in the susceptor, we achieved a large difference in the longitudinal and transverse conductivity in the susceptor, and, thereby, tailored the current density within the susceptor. A tailored susceptor having openings with a length (L) to width (W) ratio of 2:1 has a longitudinal conductivity about four times the transverse conductivity. In addition to tailoring the shape of the openings to tailor the susceptor, we altered the current density in regions near the edges by increasing the foil density (i.e., the absolute amount of metal). Increasing the foil density along the edge of the susceptor increases the conductivity along the edge and reduces the current density and the edge heating. We increased foil density by folding the susceptor to form edge strips of double thickness or by compressing openings near the edge of an otherwise uniform susceptor. We found these susceptors difficult to reproduce reliably. Also, their use forced careful placement and alignment to achieve the desired effect.
The tailored susceptor for our earlier application was designed to use with the cup core of U.S. Pat. No. 5,313,037. With this coil, the magnetic field is strongest near the edges because the central pole creates a null at the center. Therefore, the susceptor is designed to counter the higher field at the edges by accommodating the induced current near the edges. The high longitudinal conductivity encourages induced currents to flow longitudinally.